Philco 50T-1401 Ion Trap & Low Contrast
 

Hello, all....I have been on the ARF's for a few years now and have enjoyed lots of useful radio feedback and info from the great guys over there ....however I've started my 1st TV resto, so I thought it might be most helpful to post here for this set's remaining issues.

After a complete re-cap, and contacts and controls cleaning I have a pretty decent picture with the exception of a general lack of contrast, some stubborn retrace lines, and an ion trap that I can't adjust for optimum brightness.

https://www.dropbox.com/sh/vsodavszt...030988.JPG?m=1
I combed thru all the video circuits looking for out of tolerance resistors and found a half a dozen or so and replaced them, but no change, so I am shelving the contrast issue for now as I'd like to check the associated tubes before proceeding.
(I am currently out of the country, and don't have my tube tester and other test equip with me....DMM only). As a matter of interest I went thru the Sams voltage checks and found only 20VDC on the cathode of the 12LP4....Sams shows 130VDC??? The Philco factory schematic shows no voltage measurement here.

Same drill for the retrace lines...I checked the DC resto area, and found nothing amiss, so further ideas here would be welcomed.

The ion trap magnet adjustment is a bit of a puzzle....the brightest screen occurs when the trap is pulled back aginst the CRT base, but at a cockeyed angle that won't stay put....the spring tension pulls the trap back straighter as soon as I let go of it!

https://www.dropbox.com/s/7ryx2cqn29ntwuw/P1030987.JPG

As you can see, it is still tilted a bit, but the picture actually gets brighter if I twist it a bit more....it just won't stay that way. Does this seem like a defect in the internal
crt element arrangement? I thought of removing the spring and super gluing the trap back on, but that's not a very confidence inspiring fix! Ideas here as well???

Thanks!
Mark

Is the spring stuck to the glass? It should slide readily to wherever needed.

The Sylvania 12LP4 requires a double ion trap. The OEM Philco CRT used a single ion trap, which seemed a little strange, because all tube manuals specify a dual ion trap.

OK, that makes sense...I also have a faint Ion burn......pic tube replaced, 2nd trap never added!

You wouldn't get ion burn from an incorrect trap magnet. The electrons and ions shoot out the side of the neck. The magnet bends the electrons back towards the screen.

If the entire viewing area is discolored, it's probably just from years of heavy use. If you have a circle burned in the middle, it's probably been rebuilt with a straight instead of bent gun.

If your set has an AGC control try adjusting it for better contrast. IIRC the optimal setting for AGC is to turn it up until the picture looses synch and is very contrasty, then back it off a bit.

Thanks, Bob....the burn is a faint thumbprint near the center.......actually hard to see much of the time........so If the tube was rebuilt with the incorrect gun then the burn is going to continue to get worse with continued use, yes? If I can find a proper dual trap, my thought is I may be able to turn the brightness down which should weaken the retrace lines as well.

No such luck, Tom, thanks for the info for my future reference.....boy I got a bunch to learn here;)

Have you tried flipping the Ion trap front to rear? You will probably have to rotate it 180 degrees as well but it might help but I think you need a double trap also.

The retrace lines are common on older sets and are usually more prominent when the contrast is low.
I would suggest that you may have some weak tubes in the video i.f. strip, if you have any extras try replacing them one by one and see if it improves.

Thanks, Eric...I will try flipping/rotating the trap. I probably have some 6AG5's in my tube stash at home, but I may order 4 of em anyway since I have checked the vid if section pretty well at this point and I'd like to see this set working well before going back home next month.

The ion trap I mentioned in the earlier post is a single unit double ion trap. It seems like it has two different magnetic strengths. You should see a picture of one on Bob Anderson's posts of the old Admiral chassis.

I flipped the trap and it looks pretty much the same except the brightest spot is now a bit forward of the tube base, 180 deg off. It looks to me like I need to find one of the double magnet traps. The retrace lines are a bit less in this position....

https://www.dropbox.com/s/3r717qe9hn...1030990.JPG?m=

The Admiral 20B1 combo that Bob is doing looks to have 2 separate traps. If I can find a small PM somewhere, I'll experiment with a second, even if it is taped on temporary while I locate another

An interesting thread on ARF on this very subject, from which I take an incorrect or misplaced magnet would indeed cause ion burn.:scratch2:

http://www.antiqueradios.com/forums/...hp?f=3&t=12606

I concluded the opposite from reading that. Perhaps a really strong magnet could bend some of the ions towards the screen, but the set wouldn't be watchable so why would leave it running like that ?

Fair enough, so, I guess my question is, then, what caused the ion burn in the 1st place? I don't mind the spot that is there now, as it is pretty faint, but I sure don't want it to get worse just by operating the set!

My best guess is that every time the set was turned off the raster collapsed to a bright spot in the same area. Also maybe a manufacturing or rebuild defect.

Bob, I think you guessed it! On shut off, the raster first snaps to a horozontal line, which then collapses to a bright point.......(I've observed this on many old sets; filter caps in the power supply draining off?), and danged if it is not in the same neighborhood as that smudge!

So how about a piggybacked pre-switch of sorts to cut the filament to the horozontal OP tube, before powering down the whole set? Or perhaps switching the hi-voltage off somehow before turning the power switch? With good 12LP4's not extinct, but not exactly growing on trees, I'd like to preserve this one as long as possible!

Came up with a waaaay better solution......just turn the brightness control all the way down before shutting the set off......no bright spot! duh!

You can do a similar thing automatically by connecting an electroytic cap between B+ and the brightness line, if the brightness changes the voltage on the G1 of the CRT. When you turn the set off, the charge in the cap will push the brightness negative as the B+ supply collapses, thus pushing it negative, cutting off the emissions of the CRT cathode. Thus no or greatly reduced spot.

That would be a great to add feature for the set; looking at the schematic, the center wiper of the brightness control goes directly through a 100k resistor to
the cathode of the 12LP4, so not sure if that would work, however.

I never really understood the whole double ion trap "beam bender" magnet thing. I mean any tube guide shows that application on 10BP4s, but I have one with only a single trap and have seen others. I would have to take a look at my 12LP4 in a '50 RCA to see what type it has.

Back to the topic: That positioning you mention of having your trap all the way back against the tube plug is something that I have seen more than once. Either way you look at it, it's wrong. The last set that I can remember specifically the conditions that were at hand was a mid/later 50s Magnavox 21". The set worked fine till the flyback melted. I did some repair attempts at piecing together the fly as opposed to troubleshooting and repairing what was causing the fly to heat up. Long time ago.

Anyway, at some point it seemed that the set might work and was producing some level of high voltage. I STILL need a H.V. probe BADLY! LOL! The voltage was low, but for some reason of I pulled that ion trap all the way back to the plastic plug at the very end of the glass I produced a raster. Not specifically bright, but a raster.

Years later after the Internet came into being a household fixture, I was able to locate a flyback which probably never would have happened without the WWW. Put that in and changed a few caps and I had closer to correct high voltage. Again, no probe but you can sort of tell. Long story short, at that point I had to move the trap back to its correct position about .5 inches in toward the yoke.

The Sylvania data sheet for the 12LP4 does not give a position for the ion trap magnet, but the GE & Tung-Sol sheets do:

https://www.dropbox.com/s/507h6hokl4...2007.18.07.png

I'd really like to find the correct double magnet unit, but so far, no luck. Looks like the magnet should be approx 2" forward of the base key end....mine is at about 1-3/4" and is the best it gets. The search continues.....

Wow, I've never noticed a given position for a trap. I have seen approximate placement pictorials in old "how to" books and school textbooks. Really all you have to do is turn the brightness down a bit, I think it may be halfway, but I'm not positive. Really it's no matter, but when you turn that magnet around that tube neck in various spots while moving position between the yoke and the tube plug you will find your raster as there will be none with the magnet grossly mispositioned.

Once you find the raster, then you fine tune for maximum brightness while at the same time micro-adjusting for corner shadows. The proper position will be somewhere in between the yoke and tube plug; not against either.